Assuming you entered from the left, you'll be greeted by a pair of birds which fly slowly up and down. As you proceed to the right, smaller birds which fly across will appear and shoot at you. Destroy them if you can and continue until the ledge comes to an end, and drop down. A few more slowly flying birds will be there. As you continue to the right, several looping enemies will appear high in the sky and make their way down to the ground. Stand in a safe place and destroy them before moving on and exiting to the right.